Vehicle manufacturers are required to produce vehicles having on-board diagnostic systems which monitor various engine, powertrain, and vehicle systems and identify various malfunctions that lead to discernible increases in emissions. One such diagnostic system comprises an engine intake flow rationality diagnostic algorithm, which has been implemented to monitor measurable parameters which contribute to determining flow of intake air into an engine. Introduction of new intake system technology onto engines, including for example, variable valve timing systems, and cylinder deactivation systems, has complicated the ability of engine intake flow rationality diagnostic algorithms to effectively monitor and diagnose various engine components that provide input to the engine intake flow rationality diagnostic algorithm.
Measurable parameters used by the engine intake flow rationality diagnostic algorithm may include, for example, ambient air pressure, ambient air temperature, intake manifold pressure, throttle valve position, idle air control valve position, and mass airflow measurement. The algorithm monitors the specific measurable parameters, and executes one or more computer models to calculate expected values for intake airflow. The engine intake flow rationality diagnostic algorithm is able to determine whether components including a throttle position sensor (TPS), manifold absolute pressure sensor (MAP), mass airflow sensor (MAF), and idle air control device (IAC) are operating within allowable ranges, taking into account various factors. Factors that may affect output of a typical engine intake flow rationality diagnostic algorithm include operating conditions and measured parameters, and sources of variation due to component tolerances, component tolerance stack-ups, vehicle-to-vehicle variations, and, operator-to-operator variations, all occurring over a broad range of environmental and road conditions.
The engine intake flow rationality diagnostic algorithm includes a throttle model comprising a computer-executable algorithm, which calculates mass flow of intake air through the engine throttle body as a function of measured parameters, including ambient air pressure, ambient intake air temperature, intake manifold pressure, throttle valve position, and idle air control valve position, when applicable. The throttle model uses data points comprising the measured parameters, coupled with empirically determined coefficients, to calculate the mass air flow into the engine at a time-certain, i.e. a specific point in time. One coefficient is the discharge coefficient, Cd, comprising a calibratable table of discharge coefficient values correlated to discrete measures of throttle position (typically a discharge coefficient determined for every, ten percent of throttle angle) stored in the on-board computer. The throttle model is preferably executed in the on-board computer by capturing the measured parametric values, determining an applicable discharge coefficient Cd based upon throttle position, and executing algorithms comprising flow equations to determine expected engine airflow based upon the discharge coefficient and the measured parametric values. The currently employed throttle models feature discharge coefficients which are held constant at each throttle position for a given throttle body design applied to a specific engine configuration.
On-vehicle research of a fleet of vehicles has indicated that the throttle body may comprise a significant source of vehicle-to-vehicle performance variation for the aforementioned engine intake flow rationality diagnostic. This vehicle-to-vehicle performance variation is due, at least in part, to part-to-part variation in airflow across the throttle valve, determined as a function of the measured parameter, throttle position.
Therefore, what is needed is a throttle model that adapts to each individual vehicle. Adapting the throttle model to each individual vehicle requires a method to determine a plurality of discharge coefficients that are specific to an individual vehicle. The result includes reducing variation, thus improving robustness of the engine intake flow rationality diagnostic. Adapting the throttle model to each individual vehicle facilitates introduction of a robust engine intake flow rationality diagnostic on engine systems employing new air intake system technologies.